1. Field of the Invention
This invention relates to thermal ink jet printers and, more particularly, the control of ink droplets ejected from thermal ink jet printheads to enhance the quality of printing.
2. Description of Related Art
A thermal ink jet printhead selectively ejects droplets of ink from a plurality of drop ejectors to create a desired image on an image receiving member. The printhead typically comprises an array of drop ejectors that convey ink to the image receiving member. The printhead moves back and forth relative to the image receiving member to print the image. Alternatively, the array may extend across the entire width of the image receiving member. In either case, the image receiving member moves perpendicularly relative to the linear array of the printhead. The ink drop ejectors typically comprise ink passageways, such as capillary channels, having a nozzle end and are connected to one or more ink supply manifolds. Ink from the manifold is retained within each channel until, in response to an appropriate signal, the ink in the channel is rapidly heated and vaporized by a heater element disposed within the channel. Rapid vaporization of some of the ink creates a bubble that causes a quantity of ink or droplet to be ejected through the nozzle to the image receiving member. U.S. Pat. No. 4,774,530 to Hawkins shows the general configuration of a typical ink jet printhead.
The droplet ejected from the ejector to the image receiving member forms a spot of ink, which is part of the desired image. The human eye is very sensitive to changes in spot size, especially when shaded areas and graphics are being produced and especially for color printing. Therefore, uniformity of spot size of a large number of droplets is crucial to maintaining image quality in ink jet printing. If the volume of ejected droplets varies greatly within a single image, the lack of uniformity in droplet volume will noticeably affect the size of the ink spots forming the image and detract from the quality and color of the image. Similarly, if volumes of droplets ejected from the printhead differ during subsequent printings of the same image, then printing consistency cannot be maintained. Consistency is particularly important in color printing, where the resultant colors are highly dependent on the volume ratios of the ejected droplets that combine to produce the desired colors.
In addition to variations in spot size, one of the most objectionable printing defects is white striping in the image due to one or more channels of the printing device failing to operate properly. In a thermal ink jet printhead, channels can fail due to heater failure, channel plugging, air blockage in the rear of the channel, or air over the heater. Air in the channel region over the heater can occur from a variety of sources, including exsolved air from the ink, air leaks in the ink seal to the device, and air entering through the nozzle openings. Air will enter through the nozzle openings when too much ink is expelled during firing of a channel, causing air to be sucked in around the ink meniscus during bubble collapse and become trapped in the heater region or in the ink pathway leading to the heater. A thermal ink jet printhead requires that ink be in direct contact with the heater so that a vapor bubble can be formed to propel the next droplet of ink to properly function. If any significant amount of air covers the heater, the vapor bubble will not be formed properly, and the printhead will misfire. In addition, if an air bubble is trapped in the ink pathway leading to the heater, it will inhibit refill of the channel. Further, as a printhead warms up, due to changes in ambient temperature or due to heat generated by the printing process, the ink viscosity decreases. As a result, droplet volume increases with temperature so that missing droplets due to air entering the nozzles becomes more prevalent at elevated temperatures.
Several prior art devices have attempted to control the temperature of the heater to control the droplet and subsequent spot size.
For example, U.S. Pat. No. 4,980,702 to Kneezel et al. discloses a temperature control system that utilizes a control circuit that regulates heater operation to maintain the printhead in a desired operating range.
However, controlling the temperature of the heater is difficult to achieve a constant temperature range and requires large feedback time to sense the temperature, regulate the heater and check the regulated temperature.
To overcome the difficulties of directly controlling the temperature of the heater, U.S. Pat. No. 5,223,853 to Wysocki et al. proposes selectively applying an electrical input signal having an amplitude and time duration to the heater element to affect the size of the ejected ink droplet.
It is known that the size of a discharged droplet is determined by various controlling factors such as electrical energy quantity as discussed in U.S. Pat. No. 4,345,262 to Shirato et al. However, none of the prior art patents disclose a method or apparatus for reducing the occurrence of missing droplets, particularly at elevated operating temperatures.